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Sinclair is told, "There is a hole in your mind." This refers to when during the Battle of the Line, he was taken aboard a Minbari vessel, and found to contain part of the Minbari soul. The Minbari realized they were fighting a part of themselves, and surrendered immediately. It wasn't revealed why they surrendered. Sinclair had his memory wiped, and had no recollection of being on the Minbari vessel, except for some small, short, vague dreams. When Sinclair discovers the truth, it triggers the events that follow, including the events surrounding Valen, Babylon 4, and the transformation of Delenn. Once the precursors of these events are put into motion, stopping them becomes impossible. As Kosh also said, "The avalanche has started. It is too late for the pebbles to vote."